PSU and mobo problems

Hi everyone.
I have a HTPC with this PSU and this motherboard.

It's been working fine for about 15months now, but a couple of weeks ago the computer wouldn't turn on. I opened the case and watched the power lights on the psu turn on and then off again immediately when replugging the power chord. I figured it would have to be the PSU and ordered a new one.

I got the new one yesterday and I started removing the PSU from the case, unplugging everything etc. Just to be sure that it was the PSU, I reconnected the power chord to test it. And guess what, the lights turned on and it seemed to be working normally. This was without any connections to motherboard, disks etc. I then plugged in the main power to the mobo and got no lights. I hear a small buzzing actually. So... Is it the mobo or the PSU?

It's worth mentioning that the full case is completely free of fans. So it can get pretty hot inside, which probably affects the components. I've replaced the graphics card thrice due to overheating. But no other components seem to have been affected, until now that is.

The case is a nightmare working inside due to the heat pipes cooling the cpu. So I'd rather not remove the mobo until I know it is faulty. I have another computer I can plug the PSU in, but I'd rather leave it alone until I get some more input on what could be wrong. Anyone got any good tips?

Thanks